Parting Thought

A number of highly toxic plant substances manipulated by insects have been considered here. How do they do it? The tobacco alkaloids, represented by nicotine, were considered in Chapter 9. Here are a group of simple substances, highly toxic to higher animals and insects, long used as a commercial insecticide. Nicotine affects acetylcholine receptors in the central nervous system. Yet Lepidoptera of the subfamilies Macroglossinae and Sphinginae are able to tolerate large quantities of nicotine. The tobacco hornworm, Manduca sexta and the cigarette beetle Lassioderma serricorne have adapted to live only on fresh tobacco leaves or cured tobaco respectively. Manduca larvae thrive better on plants with lower nicotine levels than on those on artificially high nicotine, so the alkaloid is no help to them. Yet they prefer to feed on young leaves, and nutritionally thrive much better on them although they have twice a much nicotine in them as older leaves. How do they cope with the toxins? It has been demonstrated for M. sexta that their acetylcholine receptors are not different from those of other insects. They do not tolerate the nerve poisons, rather they appear to have a rich variety of detoxifying enzymes to break down the nicotine, particularly in their central nervous system. Certainly only 10 to 20% of nicotine injected into tolerant larvae can be recovered as nicotine, nicotine N-oxide or cotinine (Figure 9.2). What happens to the rest? We do not know.

We have, as yet, few clues to how insects overcome plant defences.

With insects, nothing can be taken for granted and their chemistry still holds many surprises.